1. Field of the Invention
The present invention relates to an optical loopback apparatus for changing the direction of incident light for evaluation of optical fibers and determination optical properties.
This application is based on patent application No.Hei09-081427 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 7 is a schematic drawing of an example of the conventional optical loopback apparatus. An optical loopback apparatus is a device to turn the direction of propagation of an incident beam of light 180 degrees back in the opposite direction, and is comprised in general of an optical fiber 51 having an optical plug 52a at one end, and another optical plug 52b at the opposite end of the fiber. Looping back of the beam is achieved by bending the optical fiber 51 in the loopback apparatus with a minimum radius of curvature R.sub.min, so that the output surfaces of the optical plugs 52a, 52b face in the same direction, i.e., to the left in the case shown in FIG. 7.
In such a loopback apparatus, in order to turn the optical fiber 51 about the minimum radius of curvature R.sub.min, it is necessary to secure a sufficient space represented by a bending range 51a shown in FIG. 7. For example, for a commonly used minimum radius of curvature R.sub.min of about 30 mm, the bending range 51a is about 60 mm which is a serious barrier to achieving miniaturization of the loopback apparatus.
FIG. 8 is a cross sectional view of another example of the conventional optical loopback apparatus. Incident beam of light is propagated through an optical fiber 56a having a ferrule 55 at one end (at right end in FIG. 8).
The beam is input into a lens 58a which converts it to a parallel beam of light which is directed into a prism 59, which reflects the input beam twice to turn the direction of propagation of the parallel beam 180 degrees. The parallel beam turned around by and output from the prism 59 passes through another lens 58b, and is injected into another optical fiber 56b having a similar ferrule 55 as that for the optical fiber 56a, and is propagated through the optical fiber 56b as an output beam.
According to the construction of the loopback apparatus shown in FIG. 8, it becomes necessary to align optical axes of the two optical paths: i.e., optical fiber 56a and ferrule 55 with the optical axis of lens 58a, on the one hand, and optical fiber 56b and ferrule 55 with the optical axis of lens 58b on the other. Therefore, it is necessary to have a mechanical arrangement such that each component in the apparatus can be independently manipulated.
Therefore, it is clear that, regardless of the type of loopback apparatus chosen, it has been difficult to reduce size or weight of the apparatus or to simplify its construction.